This invention relates to an ultrasonic detector for converting ultrasonic signals into electrical signals, particularly for measuring instantaneous intensity values at well-defined points.
With the increasing application of the ultrasonic techniques in medical diagnostic and therapy there also follows an essentially increased need for characterization of the ultrasonic transducer. It is necessary for instance to control that the acoustic energy emitted does not exceed a limit value which may be dangerous to the patient or the operator. The most customary method of performing these measurements makes use of very sensitive balances in liquid baths. These balances measure the ultrasonic pressure which is proportional to the acoustic energy emitted (Carson et al. 1978, Ultrasound in Med. & Biol., Vol. 3, pp. 341-350). For performing this type of measurement use can also be made of the heat produced at the absorption of the acoustic energy (Torr et al. 1977, Phys. Med. Biol. 22, pp. 444-450).
In the two aforementioned methods, the mean value of the acoustic energy emitted is measured. However, to obtain the relative intensity distribution or instantaneous intensity values other methods are required, one of them being the acousto-optical method. This method makes use of refraction effects or interference of optical signals with the acoustic signal (Reibold 1977, Acustica Vol. 36, pp. 214-220). Being complicated, this method is applied in special cases only.
A more interesting method is to measure with the aid of a detector device being in the form of a microprobe. The active material in such a probe is a piezoelectric material in the size of 1 mm or less, which converts the acoustic signal into an electrical signal. With such a measuring probe it is possible by simple means to measure the extension and instantaneous values of the acoustic energy from the transducer and, in certain cases, also the appearance of the acoustic signal (Lewin 1981, Ultrasonics sep. pp. 213-216).
Hitherto, the microprobes have suffered from the disadvantage of being difficult to produce because the measuring probe shall have a small active surface and internal oscillations and reflections in the probe can hardly be avoided. Such oscillations and reflections must be prevented in order that the acoustic signal shape may be registered. Besides a very good electrical shielding of the probe is necessary to prevent a disturbance of the measuring result by capacitive coupling of the excitation signal to the measuring probe. Normally, there is used for such probes a piezoelectric material which is coated with electrodes of for example nickel or aluminum on both sides, of U.S. Pat. No. 4,316,115. The above-mentioned drawback of such probes give rise to difficult insulation and connection problems because of the double-sided coating.